Lead is toxic to numerous organ systems, including the reticuloendothelial and nervous systems. The most important consequence of low-level lead toxicity is irreversible neurological damage. Although ambient lead levels are markedly below those of a decade ago as a result of the reduced use of leaded gasoline, there remain the problems of widespread detection of lead-paint poisoning of children, control of lead exposure in the workplace, increased lead absorption and low-level lead toxicity.
Recent neuroepidemiologic studies have demonstrated peripheral neurological abnormalities in lead-exposed adults at levels of 30-40 .mu.g/dl and in children at blood lead levels as low as 20-30 .mu.g/dl. Seppalainen et al. (1983) Neurotoxicology, 4:181-192; and Needleman et al. (1979) N. Eng. J. Med., 300:689-695. The Second National Health and Nutrition Examination Survey (NHANES-II) found that 1.5 million preschool children had blood lead levels of 25 .mu.g/dl and above, indicating lead absorption.
Recent evidence indicates that the prevalence of central neurological symptoms is elevated and neurobehavioral performance is impaired in adults at blood lead levels of 40-60 .mu.g/dl. Baker et al. (1985) Brit. J. Industr. Med., 41:507-516; Jeyarathnam et al. (1987) Brit. J. Industr. Med., 43:626-629; and Lilis et al. (1985) Arch. Env. Health, 40:38-47. Central nervous system dysfunction in children with elevated blood lead levels has been measured using verbal I.Q. test scores. In one U.S. study, children with blood lead levels of 25-45 .mu.g/dl scored 4.5 points lower on the I.Q. tests after adjusting for parental education, childhood illnesses and socioeconomic status. Needleman et al. (1979). In two cross-sectional European studies, similar effects of low-level lead central nervous system toxicity were observed. Winneke et al. (1981) Internat. Conf. on Heavy Metals in the Environment, Geneva, World Health Organization, pp. 553-556; and Smith et al. (1983) Dev. Med. Child Neurol., 25(suppl 47):1-20. Furthermore, both decreases in intelligence and shortened attention span have been reported in young children who had moderately elevated umbilical cord blood lead levels at birth. Bellinger et al. (1987) N. Engl. J. Med., 316:1037-1043. By all measurements, children appear to be at significant risk from low-level lead exposure. Needleman et al., (1990) N. Engl. J. Med., 322:83-88.
In a rat model, lead has been shown to be mobilized from maternal stores during pregnancy and to cross the placenta. Moreover, lead has been shown to produce neural tube defects in three rodent species. Gerber et al. (1977) Mutat. Res., 76:115-141. In man, increased placental lead concentrations have been reported in stillborns and in infants with mental retardation and congenital anomalies. Wibberly et al. (1977) J. Med. Genet., 14:339-345. A study of over 5000 births with 3% congenital malformations demonstrated a significant dose-related correlation between placental lead concentration and congenital anomalies. Needleman et al. (1984) J. Amer. Med. Assoc., 251:2956-2959. The effects of other possible teratogenic variables were eliminated by multivariant analysis.
There is also evidence that lead affects the male gamete. Increased numbers of chromosomal aberrations in sperm have been found both in experimental animals exposed to lead and in lead workers. Deknudt et al. (1973) Environ. Physiol. Biochem., 3:132-138. Lead was one of only three substances included on the initial California "short list" of reproductive toxins. Baum. Chem. Eng. News, Mar. 16, 1987, p. 22.
ALA-D, the second enzyme in the heme biosynthetic pathway, catalyzes the asymmetric condensation of two molecules of 5-aminolevulinate (ALA) to form the monopyrrole, porphobilinogen (PBG), the precursor of heme, cytochromes and cobalamins. The mammalian enzyme has been purified to homogeneity from bovine liver and human erythrocytes. Wu et al. (1974) Proc. Natl. Acad. Sci. USA, 71:1767-1770; Gurne et al. (1977) Proc. Natl. Acad. Sci. USA, 74:1383-138; and Anderson and Desnick (1979) J. Biol. Chem., 254:6924-6930. ALA-D is a metalloenzyme composed of eight identical subunits and eight zinc atoms. Anderson and Desnick (1979); Bevan et al. (1980) J. Biol. Chem., 255:2030-2035; Tsukamoto et al. (1979) Biochem. Biophys. Acta, 570:167-178; Tsukamoto et al. (1980) Int. J. Biochem., 12:751-756; and Jaffe et al. (1984) J. Biol. Chem., 259:5032-5036.
ALA-D activity is inhibited by lead and various heavy metals as well as by the oxidation of critical thiol groups. Anderson & Desnick (1979); and Jordan et al. (1985) Biochem. J., 222:1015-1020. Lead atoms replace the zinc atoms which are required to maintain ALA-D activity. Tsukamoto et al. (1979). The inhibition of erythrocyte ALA-D activity has been used as a sensitive diagnostic indicator of lead exposure. Morgan et al. (1972) Arch. Intern. Med., 130:335-341. The inhibition is stoichiometric; e.g., 15 and 30 .mu.g of Pb per dl blood results in 50% and 75% ALA-D inhibition, respectively. More recently, the ratio of ALA-D present before and after reactivation with zinc and DTT has been shown to correlate best to blood lead levels. Chisholm et al. (1985), Clin. Chem., 31:662-668. The inhibition of ALA-D activity results in a proportionate accumulation of ALA blood and urine. The accumulation of ALA has been causally related to the neurological manifestations of lead poisoning.
Human ALA-D, has been shown to be a polymorphic enzyme. Petrucci et al. (1982) Hum. Genet., 60:289-290; and Battistuzzi et al. (1981) Ann. Hum. Genet., 45:223-229. The allelic polypetides are encoded by a gene located on chromosome 9 in the region 9q34. Potluri et al. (1987) Hum. Genet., 76:236-239. The ALA-D gene has two common alleles, ALA-D.sup.1 and ALA-D.sup.2, which result in a polymorphic enzyme system with three distinct charge isozyme phenotypes, designated ALA-D 1-1, ALA-D 1-2 and ALA-D 2-2. The isozymes may be separated by starch gel electrophoresis. Battistuzzi (1981). In the Italian population, the frequencies of the isozyme phenotypes are 1-1 (81%), 1-2 (17%) and 2-2 (2%), consistent with gene frequencies of 0.90 and 0.10 for the ALA-D.sup.1 and ALA-D.sup.2 alleles, respectively. Similar results were obtained in other European populations, whereas expression of the ALA-D.sup.2 allele was not observed in a large sample of Black individuals from Liberia. Benkmann et al. (1983) Hum. Hered., 33:62-64. A study of ALA-D isozyme phenotypes in erythrocytes of over 950 normal Caucasian individuals from New York showed the frequencies of the ALA-D 1-1, 1-2, and 1-2 isozyme phenotypes to be similar to those observed in the Italian population.
The occurrence of the ALA-D polymorphism is of interest, particularly with respect to the possible increased susceptibility of certain isozyme phenotypes to the detrimental effects of lead exposure. Polymorphisms at other genetic loci are known to be related to differential inherited responses to environmental challenges. For example, the response to Plasmodium (malaria) is affected by hemoglobin S, hemoglobin AS heterozygotes being more resistant to disease than individuals with normal hemoglobin AA. Pasval et al. (1978) Nature, 274:7801-7803. Similarly, Asian individuals are more susceptible to alcohol intoxication due to the presence of a particular alcohol dehydrogenase polymorphism. Propping (1978) J. Physiol. Biochem. Pharmacol., 83:124-173.
The existence of this common ALA-D polymorphism and the fact that ALA-D is markedly inhibited by lead suggests that there is a physiologic relationship between the frequency of the ALA-D.sup.2 allele and lead poisoning. For instance, individuals with the ALA-D.sup.2 allele may be more susceptible to the detrimental effects of lead exposure if the ALA-D.sup.2 subunit bound lead more tightly than the ALA-D.sup.1 subunit. They would have higher blood and bone lead concentrations as well as higher total body lead stores, making them even more likely to express subclinical and clinical manifestations of chronic low level or acute lead exposure. Alternatively, the tight binding of blood lead by erythrocyte ALA-D.sup.2 may prevent the distribution of lead to the neurologic system, thereby preventing or minimizing the neurotoxic effects of lead.
A study of blood lead levels and ALA-D isozyme types in 1277 blood samples obtained from the New York City Lead Screening Program was performed in a double-blind fashion. That is, the blood lead levels were provided by the Toxicology Laboratory only after the blind determination of the ALA-D isozyme phenotype. Table I is a compilation of the number of individuals with the ALA-D 1-1, ALA-D 1-2, ALA-D 2-2 phenotypes having blood lead levels above or below either 25 or 30 .mu.g/dl. These results demonstrate that a high proportion of these individuals with high blood lead levels had the ALA-D.sup.2 allele. Astrin et al. (1987) Ann. N. Y. Acad. Sci., 514:23-29. The presence of the ALA-D.sup.2 allele apparently leads to approximately a two-fold increase in lead retention at blood levels of 25 or 30 .mu.g/dl. In some cases, the ethnic group was known. The incidence of the ALA-D.sup.2 allele among lead poisoned Black children was high even though the incidence of the ALA-D.sup.2 allele among Blacks in general is low. The results obtained support a relationship between the ALA-D.sup.2 allele and the accumulation of lead in the blood. Similar data support the identical conclusion. Ziemsen et al. (1986) Int. Arch. Occup. Environ. Health, 58:245-247.
TABLE 1 ______________________________________ HUMAN ALA-D POLYMORPHISM: ASSOCIATION WITH LEAD POISONING Blood Lead ALA-D Isozyme Phenotype Sample Level (Number and Percent in Sample Set) Set (.mu.g/dl) Total 1-1 (%) 1-2 or 2-2 ______________________________________ Total: &lt;25 870 803 (71) 67 (47) .gtoreq.25 408 333 (29) 75 (53) 1278 1136 (100) 142 (100) &lt;30 1000 919 (81) 81 (57) .gtoreq.30 278 217 (19) 60 (43) 1278 1136 (100) 142 (100) Blacks: &lt;30 292 282 (88) 10 (38) .gtoreq.30 53 37 (12) 16 (62) 345 319 (100) 26 (100) ______________________________________
It has previously been found that the frequency of the ALA-D.sup.2 allele to be 10-11% in Italian and German populations. Battistuzzi et al. (1981) ".delta.-aminolevulinate Dehydratase: A new Genetic Polymorphism in Man", Ann. Hum. Genet., 45:223-229; and Benkmann et al., (1983) "Polymorphism of Delta-aminolevulinic Acid Dehydratase in Various Populations", Hum. Hered., 33:62-64. A study indicated an ALA-D.sup.2 allele frequency of 20% in Eastern European Ashkenazi Jews living in Israel. Ben-Ezzer et al., (1987) "Genetic Polymorphism of Delta-aminolevulinate Dehydratase in Several Population Groups in Israel", Hum. Hered., 37:229-232. The blood samples used for this study came from a Tay-Sachs screening program, which would be biased toward an Ashkenazi Jewish population. In this experiment the observed ALA-D.sup.2 allele frequency of 12% was thus somewhat higher than the average frequency of 10-11% in European populations.
It would be useful to have a rapid, inexpensive diagnostic method to detect the human ALA-D polymorphism. Such a method is useful in screening for persons susceptible to lead poisoning so that they could be given jobs with less exposure to lead. Also, screening may identify children who are more susceptible to lead poisoning and therefore dictate a more rigorous prevention program, monitoring and/or detoxification therapy in such children.